I don't believe that providers ever change the format of the picture (at least not intentionally - if the format is incorrect, contact the broadcaster and copy the provider). From my experience, it's always the broadcaster and most videophiles want their programmes to be OAR, even if it means 16:9 inside 4 black bars on an HD channel if the original programme was SD. That way I can decide if I wish to zoom/stretch the programme which results in poor PQ and distortion if stretched. Sometimes the broadcaster may do a good job of the upconversion/zooming of 16:9 SD, so that is something that I don't worry about as much as stretching.

Of course some people zoom or stretch using their STB or their TV and that's fine if they wish to do so, it doesn't affect me. Here's the FAQ on Stretch Modes:

http://www.digitalhome.ca/forum/showthread.php?t=76074